In order to improve signal-to-noise ratio (SNR) of a class D amplifier, a tri-level current digital-to-analog converter (DAC) is used in a circuit of an audio speaker because most current DAC cells are not connected to class D amplifier when a signal is small. However, when the tri-level current DAC is applied, an additional circuits such as a current-to-voltage converter and a programmable gain amplifier is required to positioned between the tri-level current DAC and a class D amplifier for signal conversion and DC-level shifting, and these additional circuits may contribute extra noise to the signals.
In addition, in a driving stage of the circuit of an audio speaker, a fast slew rate usually leads to less distortion, better efficiency and less switching loss, however, the fast slew rate may also induce severe Electromagnetic interference (EMI) problem.